1. Field of the Invention
The present invention relates to an element isolation technology in a method for manufacturing a semiconductor device.
2. Description of the Related Art
In a conventional method for manufacturing a semiconductor device, there is a technology of realizing flatness and miniaturization of a surface of a semiconductor layer by use of a STI (shallow trench isolation) method instead of a LOCOS (local oxidation of silicon) method. A groove formed by the STI method is filled with an insulating film, and a thermal oxide film is formed on a sidewall of a trench formed from an upper surface of the insulating layer. Thereafter, by use of a CVD (chemical vapor deposition) method, the trench is filled with a CVD oxide film. Thus, an isolation region is formed. This technology is described for instance in Japanese Patent Application Laid-open No. Hei 9-8119 (p. 7 to 9, FIGS. 2 to 10).
As described above, in the conventional method for manufacturing a semiconductor device, after a thermal oxide film and a silicon nitride film are selectively formed on a surface of an epitaxial layer, a first shallow groove is formed by use of the STI method. Thereafter, a CVD oxide film is formed on the epitaxial layer so as to fill up the first shallow groove. Subsequently, a first round of CMP method (chemical mechanical polishing) is performed to etch back the CVD oxide film and form a flat surface. Accordingly, a second deep groove is formed from an upper surface of the CVD oxide film, and an element isolation region is formed.
Next, after a thermal oxide film is formed first on a sidewall of the second deep groove, a CVD oxide film is formed on the epitaxial layer, and the second deep groove is filled with the CVD oxide film. Thereafter, by use of a silicon nitride film as a stopper film, a second round of CMP method is performed to etch back the CVD oxide film. Subsequently, after the silicon nitride film used for the STI method is removed by etching, the thermal oxide film below the silicon nitride film is removed.
In this event, by use of the conventional method for manufacturing a semiconductor device, a part of the CVD oxide film in the second deep groove is also removed when the thermal oxide film is removed due to an etching rate ratio between the thermal oxide film and the CVD oxide film. Particularly, in the isolation region, a concave part is formed with respect to the surface of the epitaxial layer in the other region. Thus, there is a problem that it is difficult to form a passive element on an upper surface of the isolation region in which the concave part is formed. Moreover, there is a problem that the concave part causes a wiring layer formed on the upper surface of the isolation region to be easily disconnected.